keyframes disappeared?
What you have done is to convert the frames in an "Action Strip" or "Action", to edit this frames select the "Action Strip" (the yellow one) and press Tab to edit it. I don't know if you can revert the "action strip" to "frames"





Also if you added another keyframe, the upper frame override the bottom one. What you need to do is to convert the new keyframes in "Actions Strip" and play with their properties.

Look at image 3 and search for the blending property, it define how it will apply over others "actions". After convert "frames" to "Actions" you can drag and drop to order the "Actions", start and end times, etc.